Federal safety regulators are looking at a variety of potential solutions to the unintended acceleration issue at the root of Toyota's recall of nearly 6 million vehicles in the U.S., but won't know more about the problem's cause until this summer at the earliest, said David Strickland, administrator of the National Highway Traffic Safety Administration.

"We are looking at one-button on-off controls, smart brakes and electronic data recorders," said Strickland, who spoke Thursday at the Society of Automotive Engineers 2010 World Congress at Cobo Center.

A group of NASA scientists is exploring electromagnetic interference, software coding and cosmic rays as possible factors in unintended acceleration. They are expected to report to NHTSA this summer. Another group, from the National Academy of Science, has started a 15-month research project looking at similar issues.

Toyota executives have said two mechanical problems are causing the problem, which more than 1,100 owners have reported to NHTSA and is cited in hundreds of wrongful-death and injury lawsuits.

The automaker blames out-of-place floor mats that become trapped beneath the gas pedal, or gas pedals that stick in a partially engaged position because of excess friction. Toyota is repairing or replacing gas pedals on 2.3 million vehicles it recalled in January.

Toyota has until Monday to pay or appeal a $16.4-million fine, the largest NHTSA has ever issued, for delaying that recall after receiving reports of the problem.

Separately, Toyota said Thursday it is testing all Toyota and Lexus SUVs to reassure buyers of their safety after Consumer Reports warned its readers not to buy the Lexus GX460 because of a tendency to roll over in tests the magazine's staff conducted.

Lexus dealers in the U.S. have stopped selling the SUV until company engineers can address Consumer Reports' concerns.